Alignment devices of this type are preferably used as components of terminals which are employed for the reciprocal data transmission by means of optical connections in space. The alignment devices are used to align a transmitting beam emanating from a terminal with a beam emanating from a counter terminal and received by the first terminal. In this case the transmitting beam of the counter terminal is the receiving beam of the first terminal and vice versa, since the two terminals in principle perform the same functions and are therefore also essentially designed the same. It should be noted at this time that the terminals can also be earthbound installations, however, the advantages achieved by means of the instant invention are of importance in connection with terminals of the space satellite type in particular.
For several reasons the mentioned optical space connections are more advantageous for the transmission of data between two terminals than the directional microwave radio connections which are also employed for the purposes mentioned.
By means of optical space connections it is possible in particular to realize receiving and transmitting devices of low mass. Thanks to the comparatively short wavelength of light it is possible to emit an optical beam by means of an optical device of relatively small size over a narrow spatial angle. By means of the antenna gain obtained in this way it is possible to transmit high data rates even with a low transmission output. In contrast to this, the directional antennas required for directional microwave radio connections have a comparatively large size and mass, which represents an important disadvantage, particularly in connection with satellite terminals.
The advantageous option of making use of an optical device of small dimensions and therefore of low mass are based on the good collimating capability of optical beams, which surpasses the collimating capability of microwaves.
For one, the demands made on the accuracy of the alignment of the transmitting and receiving beams are high, because of the tight collimation of the beams or their emission over a narrow spatial angle range, and are also not easily met. This manifests itself already during the establishment of a connection between two satellite terminals, which does need to be made by the terminals themselves. Even after the establishment of a good connection, the collimation of the optical transmitting beam requires an appropriately exact alignment of the respective transmitting and receiving beams at every moment, or the appropriate tracking of the devices used for transmitting and receiving the beams.
An additional difficulty lies in that tracking requires not only comparatively continuous movements, but also that, for compensating mechanical vibrations which occur over a relatively wide frequency range and are caused by a terminal support or the satellite, the alignment device must also be able to perform corresponding high-frequency compensatory vibrations. The terminal is mounted on a support platform of the satellite; the latter moves freely in space and is not supported in any way, because of which it is not only not connected with any additional physical element, but is also not surrounded by an atmosphere. Therefore possibly occurring mechanical vibrations are not transmitted to the environment, such as is customary in connection with terrestrial installations. Because of this, there is the danger that the satellite, and of course the terminal along with it, is set to vibrate mechanically, namely by shocks induced, for example, by rocket drives, which are put into operation for orbit changes and orbit corrections, and by moving elements, such as stabilizing devices. These mechanical vibrations are of course transmitted to the terminal and thus to all components disposed in the terminal, therefore also to the components intended for the optical data transmission, by which the correct alignment of the transmitting beams or maintaining the receiving direction can be hampered, which requires an appropriate alignment later. Although this problem occurs in space in general, it has particularly grave consequences in connection with data transmission by means of optical beams because of their tight collimation.
There are several different partial operations within the totality of the alignment processes. When establishing a connection, i.e. prior to the start of data transmission, it is initially necessary to detect either a tightly collimated search beam emitted by the counter terminal, which possibly illuminates a relatively large spatial angle, or suitable natural light sources, for example constellations of stars, must be detected, which permit the highly accurate determination of their own position in space. Secondly, an accurate alignment of the transmitting beam with the search beam, or in relation to the spatial direction in respect to the mentioned light sources, must take place, also prior to the start of the data transmission. A third necessity is the permanent tracking of the alignment during the data transmission in order to be able to compensate erroneous deviations from the highly precise alignment, which can have several causes.
Furthermore, it is necessary in general during the alignment to take a relative movement between the terminal and a counter terminal into consideration. Up to now it was always assumed that no relative movement between the terminal and the counter terminal takes place. In this case the aim of the alignment is limited to aligning the transmitting beam exactly with the receiving beam. However, if there is a relative movement between the terminal and the counter terminal, the direction of the transmitting beam must slightly deviate from the receiving direction, i.e. it is necessary to include the keeping of a lead correction angle in the alignment. It is possible to omit the realization of a lead correcting angle, if the opening angle of the transmitting beam is greater than the required lead correction angle. In this case this omission must be paid for by an increased output demand on the transmitting laser, while maintaining the same output reserve.
The demands which are made on the alignment device are therefore very extensive; on the one hand it is necessary for a movement over a large spatial angle, approximately in the range of a hemisphere or possibly more, to take place, in order to detect a receiving beam at all even with an unfavorable placement; on the other hand, a very rapid movement must be possible in order to compensate even high-frequency vibrations of the receiving beam during tracking.
Since obviously an efficient device for the tracking of transmitting beams for the purpose of compensating high-frequency vibrations should, for dynamic and energetic reasons, have only a minimal mass which needs to be moved, but the total mass of the alignment device can not be arbitrarily minimized, a division of work and therefore also a division of the alignment device into two partial devices, which are intended to meet the two mentioned demands, was provided in a device of the type mentioned at the outset, which is described in FR-A1 2 690 532. A first partial device, which contains the largest portion of the mass of the entire alignment device and which is identified as a rough alignment device, permits movement over a wide spatial angle up to the range of a hemisphere, but without the possibility of performing high-frequency mechanical vibrations, so that it is not possible by means of this rough alignment device to provide a compensation of high-frequency mechanical vibrations of the terminal or satellite. The known rough alignment device is designed in the manner of a periscope and has two mirrors, wherein the transmitting ray is sequentially reflected at the first and second mirrors prior to being emitted. The direction of the transmitting beam is finally determined by the position of the second mirror, which can be rotated around a second axis in relation to the first mirror, while the first mirror, together with the second mirror, can be rotated around a first axis, and wherein the two axes extend orthogonally in respect to each other. A second partial device, whose mass should include only a very small portion of the entire alignment device and which is identified as the fine alignment device, is arranged on the first partial device, i.e. the rough alignment device, and moves together with it. In addition, the fine alignment device can move relative to the rough alignment device, namely at a high mechanical frequency but only within a very limited spatial angle.
A great disadvantage of the alignment device described by FR-A1 2 690 532 lies in that its total mass is large, wherein the mass moved by the fine alignment device in particular is too large for efficient tracking, even though it only contains one of the mirror devices.